The technology disclosed herein generally relates to laser-based instruments used to measure the three-dimensional (3-D) coordinates of a point. More particularly, the technology disclosed herein relates to systems and methods for measuring the angular position of a laser beam being emitted by a laser-based coordinate measurement device (such as a laser tracker). As used herein, the term “laser” means a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.
When manufacturing parts, it is common practice to measure the physical characteristics of the part. These measurements can be used to determine whether the manufactured part is within specified tolerances or allowed thresholds for the part. A coordinate measuring machine can be used to measure the physical characteristics of a part. This type of machine may be manually and/or computer controlled. A coordinate measuring machine typically has three mutually orthogonal axes that form a 3-D coordinate system. The measurements may be used to determine the coordinates of various points on a surface of the manufactured part.
A laser tracker is one type of coordinate measuring machine that measures the 3-D coordinates of a target point by emitting a laser beam toward the target point and then detecting light returned from the target point. A typical laser tracker incorporates a gimbaled mechanism that steers the laser beam to the target point. The laser beam may impinge directly on the target point or on a retroreflector or plane mirror target located at the target point. In each case, the laser tracker determines the coordinates of the target point by measuring the distance to the target point and the azimuth and zenith angles of the laser beam relative to the coordinate system of the laser tracker. The distance is typically measured using an absolute distance meter or an interferometer or both. The azimuth and zenith angles are typically measured using respective angular encoders which are attached to the mechanical axes of the laser tracker. The one distance measurement and two angle measurements are usually sufficient to locate the target point in the coordinate system of the laser tracker. If the location of the laser tracker relative to the frame of reference of the manufactured part is known, a coordinate transformation matrix can be used to convert the location of the target point in the frame of reference of the laser tracker to a location of the target point in the frame of reference of the manufactured part.
The optical and electronic components of a typical laser tracker are well known. For example, laser trackers having various optical systems for steering a laser beam and receiving returned light for the purpose of measuring a distance are disclosed in multiple U.S. patents issued to Faro Technologies, Inc., Lake Mary, Fla. Such components, being well known in the art, will not be described in detail herein.
Laser trackers, although quite accurate over short distances, become increasingly less accurate over distance, to the point where error on the order of ±0.005″ or greater is often present in the measurement of an object far away (50 feet or greater). In most applications in which laser trackers are used, this is not a problem, as the structures to be measured are relatively small and the assembly tolerances are relatively loose. In aerospace, however, structures are very large and assembly tolerances are very tight. Because the laser trackers used to locate aircraft assemblies in a factory are themselves not accurate over long distances, an aircraft manufacturer may compensate for this error by (a) mandating tighter manufacturing tolerances upstream of assembly, which adds cost, or (b) utilizing very large, expensive tooling and/or gantry machines that constrain the assemblies accurately with respect to features close to the laser tracker that can then be accurately measured. This also adds cost, in addition to taking up a large amount of factory floor space and making the factory difficult to reconfigure as production rates and the product being manufactured change.
Most laser trackers use three measurements to calculate the position of an object in space: a distance measurement taken using a laser interferometer; and two angle measurements taken using respective angular encoders. The distance measured is the distance separating a reference point inside the laser tracker from the target point. The angles measured are the azimuth and zenith angles (sometimes referred to as the “pan and tilt angles”). The angle measurements measure the angular positions of the laser beam emitter relative to the azimuth and zenith (i.e., elevation) axes respectively. The distance measurement may have an accuracy on the order of ±0.5 micrometers/meter. The azimuth and zenith angle measurements are less accurate than the distance measurement and are a significant source of laser tracker measurement error. The angular encoders measure angles with an accuracy on the order of ±15 micrometers+6 micrometers/meter (i.e., per meter of distance to the target object). Combining the distance and angle measurements allows the laser tracker to find a point in 3-D space. The problem with this design is that the 3-D positional measurement error created by the angular measurement error may be substantial over large distances. Even if the laser beam is a tenth of a degree off from the measured angle, significant random 3-D positional error (on the order of 0.005″ or greater) is present over a large measurement volume.
In view of the foregoing, any technological advance that improves the accuracy of measurement of the angular position of a laser beam emitter (especially one incorporated in a laser tracker) would be advantageous.